The present invention relates to paper making machines and, more particularly, to an apparatus and associated method for drying a wet web of paper.
Generally, in a paper making machine, a wet paper web is formed in a former on a carrying fabric and then moved downstream. As the web is transported downstream in the paper making machine with a drying fabric, it is processed through a dewatering or drying section where it is partially dewatered or dried. In some configurations of paper making machines, the carrying fabric may also comprise the drying fabric. In alternate configurations of paper making machines, the drying fabric may be a separate fabric from the carrying fabric where the formed paper web is transferred from the carrying fabric to the drying fabric for transportation through the dewatering or drying section.
The dewatering or drying section may include, for example, one or more of a through-air dryer (TAD), an infrared dryer, an impingement dryer, a cylindrical contact dryer, or the like. The web and fabric may then be passed downstream through a nip where the web is transferred to a Yankee dryer from the fabric. Generally, where a Yankee dryer follows the dryers in the preceding section, the dryers in the preceding section are regarded as pre-dryers for partially drying or dewatering the paper web. These pre-dryers may further be regarded as comprising a part of the drying section or collectively forming a separate pre-drying or dewatering section. The Yankee dryer is accordingly regarded as the final dryer for drying the paper web. However, where the paper making machine does not include a Yankee dryer, the dryers in the preceding section comprise the final dryers for drying the paper web.
As the paper web proceeds from the former to the final dryer, the processes therebetween often leave residue from the paper web on the fabric or fabrics used to transport the paper web through the paper making machine and, particularly, through the dewatering and/or drying section. Since fabrics used for transporting the web are typically configured as endless loops, they continuously cycle about the forming and/or drying processes as the paper is being produced and dried. Thus, for proper operation of the paper making machine, the web-carrying fabrics should be cleaned before they cycle back to receive more of the paper web to be transported through a particular process. Cleaning the fabric often consists of washing the fabric by passing it under a shower and then partially drying the fabric, for example, by passing it by a vacuum box. However, some special types of paper making processes involve special fabrics, such as TAD fabrics or other texturing fabrics used in tissue manufacturing which typically have an open structure and which also require cleaning through the entire thickness of the fabric, that require special and often complicated cleaning equipment.
A paper making machine thus generally comprises one or more fabrics carrying a paper web, a drying section where the web is dried by one or more dryers, a cleaning section where the fabric is cleaned before cycling back to receive more of the wet paper web from a web forming section, and a reel-up for receiving the dried paper web and winding it onto a spool. Accordingly, the paper making process is typically accomplished by having the drying section disposed underneath the cleaning section with the drying fabric loop running therebetween. At one end of the loop, the paper web is formed on or transferred to the drying fabric in the forming section. At the other end of the loop, the fabric passes through the dewatering or drying section and/or the nip at the Yankee dryer. Where a Yankee dryer is used in a paper making machine, the rotations of the loop and the Yankee dryer (which is a rotatable drying cylinder) are coordinated such that the web is transferred to the Yankee dryer and carried over the top thereof in an upright orientation before being creped by the doctor blade. Creping the web in an upright orientation requires the web leaving the pre-drying section to encounter an upwardly moving surface of the Yankee dryer at the nip. Once dried by the Yankee dryer and creped, the dried web is transported to the reel-up to be wound onto a spool.
Prior art paper making machines exhibit some disadvantages resulting from the basic layout described. For example, the typical configuration wherein the cleaning section is above the drying section means that, for the most part, the wet web is transferred from the forming section to the fabric, and transported through the drying section to the nip, on the bottom surface of the fabric. That is, the web is in contact with the fabric, but the fabric does not support the web against the force of gravity. As such, there exists the possibility that the web may fall off the fabric during this xe2x80x9cinvertedxe2x80x9d web run. In addition, with the cleaning section typically disposed above the drying section, elaborate catch pans and measures to prevent condensation must be implemented to prevent cleaning water from dripping from the cleaning section onto the underlying drying section. Water dripping from the cleaning section generally poses a problem in paper making processes involve special fabrics, such as TAD fabrics or other texturing fabrics used in tissue manufacturing which typically have an open structure and are more sensitive to water drip. These disadvantages are inherent in these prior art paper making machines and apply regardless of the type of dryer used in the drying section.
Further, the dewatering or drying section may include, for example, one or more through-air dryers (such as a flat bed dryer, a rotary roll dryer with inward air flow, or a rotary roll dryer with outward air flow) for dewatering or drying the web. Where a through-air dryer is present, a hood is also generally included and typically covers the travel path of the web about the dryer. Accordingly, in prior art paper making machines employing an inverted web run and a through-air dryer having the hood located underneath, there exists the possibility of the web falling off the fabric, jamming in the hood, and thereby causing damage to the dryer.
An example of a prior art paper making machine as described above is found in U.S. Pat. No. 5,611,890 to Vinson et al. which discloses a paper making machine, wherein the wet paper web is transferred to the fabric from a former. The web is then carried underneath and past a vacuum dewatering box and blow-through predryers before being directed upward and transferred to the Yankee dryer at the nip. The fabric then continues upward to be cleaned and dewatered, thus completing its loop by passing over and around showers and a vacuum dewatering box. Similarly, U.S. Pat. No. 5,529,664 to Trokhan et al., U.S. Pat. No. 5,776,307 to Ampulski et al., U.S. Pat. No. 5,364,504 to Smurkoski et al., U.S. Pat. No. 5,701,682 to Chuang et al., and U.S. Pat. No. 5,700,352 to Vinson et al. all disclose paper making machines similar to that described in the Vinson et al. ""890 patent.
Thus, it would be desirable to provide a paper making machine configured such that the wet paper web is transported on the fabric and through the drying section on an upper surface of the fabric, such that the fabric supports the web and lessens the possibility of the web falling therefrom. In addition, it would be desirable to provide a machine configured such that the cleaning section is not located above the drying section, thus obviating the need for elaborate means for preventing the cleaning water from dripping from the cleaning section onto the underlying drying section. It would be further desirable to provide a paper making machine with a non-inverting web run through the drying section having a through-air dryer with a hood located thereunder, such that the web will not likely jam in the hood and cause damage to the dryer.
The above and other needs are met by the present invention which, in one embodiment, provides a paper making machine comprising a water-removing section for increasing the dry solids content of the wet paper web, and a continuous fabric forming a loop such that the fabric passes through the water-removing section, the fabric loop being configured to receive the wet paper web at a web-receiving region of the fabric loop located before the water-removing section and to support and transport the web along an upper portion of the fabric loop through the water-removing section such that the web is generally on an upper surface of the fabric. The fabric loop further includes a web-transfer point at which the web is separated from the fabric and a return run over which the fabric travels from the web-transfer point to the web-receiving region. The apparatus also includes a cleaning section for cleaning the fabric, the cleaning section being located along the return run of the fabric loop and being disposed such that the cleaning section is not above the upper portion of the fabric loop on which the web is supported. Thus, any water that may drip down from the cleaning section will not drip onto the web or the part of the fabric loop on which the web is supported as it is carried through the water-removing device or devices.
In certain preferred embodiments of the invention, the water-removing devices can include pre-dryers for partially drying the wet web, followed by a final dryer for performing final drying of the web. For example, one or more through-air dryers can be provided for pre-drying the web. Final drying can be performed on a Yankee dryer, in which case the web is transferred from the continuous fabric onto the Yankee dryer by passing the fabric and web through a nip formed between a transfer roll and the Yankee dryer. Advantageously, the fabric, after passing through the nip, can be routed so as to travel a return run that includes a generally downward-running portion from the nip and a generally horizontally running portion leading back toward the web-receiving region of the fabric loop. The cleaning section preferably is disposed along one or both of the downward-running portion and the horizontally running portion of the return run of the fabric loop.
In other preferred embodiments of the invention, the water-removing devices can comprise one or more non-compacting dryers, such as through-air dryers, impingement dryers, infrared dryers, contact dryers, or the like, and the Yankee dryer can be eliminated. In this case, the fabric carries the web through the non-compacting dryer or dryers, and then the web is separated from the fabric at a web transfer point of the fabric loop. The web can then be passed to further devices, such as reel-up for winding the web into a roll. The fabric travels from the web transfer point along a return run back to the web-receiving portion of the loop. The return run can include a downward-running portion and a horizontally running portion, and the cleaning section preferably is located along one or both of said portions. Where the web is to be wound into a roll, a reel-up can be provided. The reel-up advantageously can be located above the upper portion of the fabric loop, below the return run of the fabric loop, or beside the fabric loop after the web-transfer point.
Thus, it will be appreciated that the invention enables a wet web to be supported on an upper surface of a continuous fabric loop and carried on the fabric through dryers and/or other water-removing devices, and the fabric can be cleaned after the web is transferred from the fabric, in such a way that the cleaning section is not located above any part of the web. Accordingly, there is no need for elaborate means for preventing the cleaning water from dripping from the cleaning section onto the web, as is necessary in prior art paper making processes employing a cleaning section above the drying section. The elimination of the possibility of dripping onto the fabric and web is especially advantageous in paper making machines employing special texturing fabrics or through-air drying fabrics. In addition, since the web is transported on generally an upper surface of the fabric through the water-removing devices, the sense of the rotation of the fabric is reversed with respect to prior art devices in which the web is carried by a fabric over an inverted run of the fabric. In preferred embodiments of the invention employing a Yankee dryer for final drying, the reversed rotation of the fabric enables the sense of rotation of the Yankee dryer to also be reversed relative to conventional machines, and this in turn enables a reel-up to be located above the upper portion of the fabric loop on which the web is carried, if desired. Further, since the web is carried on top of the fabric in a non-inverting web run, even where the drying section includes a dryer with a hood located thereunder, the web is not likely to separate from the fabric and jam in the hood, thereby lessening the risk of damage to the machine. It will be recognized, therefore, that the invention facilitates the achievement of a number of distinct advantages over prior paper making devices.